Variable valve timing apparatus

ABSTRACT

In a variable valve timing apparatus having a VVT mechanism in each of a plurality of banks, start of a valve timing control is not permitted until reference position learning of valve timing is completed in the corresponding bank and in other banks. Accordingly, the plurality of banks come to have matching valve timing setting at the time of valve timing control, whereby variation in the amount of air introduced to cylinders can be suppressed, and satisfactory combustion characteristics can be maintained.

TECHNICAL FIELD

The present invention relates to a variable valve timing apparatus and,more specifically, to a variable valve timing apparatus having achanging mechanism for changing a phase of opening/closing at least oneof an intake valve and an exhaust valve in each of a plurality ofcylinder groups (a plurality of banks).

BACKGROUND ART

A VVT (Variable Valve Timing) apparatus has conventionally been knownthat changes the timing at which an intake valve or an exhaust valve isopened/closed, that is, the opening/closing phase (crank angle)according to an operating condition. Generally, in the variable valvetiming apparatus, the phase is changed by rotating a camshaft, whichopens/closes the intake valve or exhaust valve, relative to a sprocketor the like. The camshaft is rotated by an actuator such as a hydraulicor electric motor.

In an engine having a configuration with a plurality of cylinder groups(banks), the VVT mechanism is provided in each bank. Therefore, when theamount of air introduced to the cylinders differs bank by bank in suchan engine configuration, variation of engine rotation (variation inrotation speed while the crankshaft rotates once) tends to occur, whichmay lead to larger engine vibration. Therefore, it is necessary toadjust setting of valve timings among the cylinder groups (banks).

In this respect, Japanese Patent Laying-Open No. 2003-172160 (PatentDocument 1) discloses a variable valve timing control device of aninternal combustion engine that enables matching of responsiveness tovalve timing control among a plurality of cylinders, even if there isimbalance in camshaft load torque among the plurality of cylinders dueto difference in the type of accessories. Particularly, Patent Document1 addresses the design in which accessories such as a fuel pump(particularly a high-pressure pump supplying fuel to an in-cylinderdirect injector) and a vacuum pump are driven only by the camshaft onthe cylinder group of one side (bank of one side). Specifically, inconsideration of a delay in valve timing control of a specific cylindergroup caused by the load of accessories, the amount of control of avalve timing adjusting portion for the specific cylinder group and/orthe other cylinder group is corrected so that the responsiveness tovalve timing control of the specific cylinder group matches theresponsiveness to valve timing control of the other cylinder group.

In order to accurately control the valve opening/closing phase (valvetiming) using a variable valve timing apparatus, it is necessary toprevent error in detecting the actual phase of valve opening/closing. Inorder to reduce the detection error, it has been a common practice toset the valve opening/closing phase at a prescribed reference positionthat is limited mechanically, and to learn the error in the detectedvalue of valve opening/closing phase at that time as an offset (see, forexample, Japanese Patent Laying-Open No. 2004-156461).

In a variable valve timing apparatus disclosed in Patent Document 2(Japanese Patent Laying-Open No. 2004-156461), reference position of thevalve timing is learned under prescribed learning conditions (forexample, every time engine operation starts), to ensure detectionaccuracy of the actual valve timing. Further, according to thedisclosure, when learning is not complete, it is determined that thedetection accuracy is low, and the rate of change in valve timing islimited. Consequently, damage to the apparatus caused by a movableportion hitting a stopper or the like at high speed can be prevented.

As described above, in the engine having a configuration with aplurality of cylinder groups (banks), consideration is necessary tomatch valve timing settings among the cylinder groups (banks) at thetime of valve timing control also during such reference positionlearning as described in Patent Document 2. Patent Document 2, however,is silent about how to execute the reference position learning for thevariable valve timing setting apparatus provided in the engine of such aconfiguration.

DISCLOSURE OF THE INVENTION

An object of the present invention is, in a variable valve timingapparatus having the VVT mechanism provided in each of a plurality ofcylinder groups (banks), to execute appropriate reference positionlearning of the valve timing such that variation in the amount of airintroduced to the cylinders among the cylinder groups (banks) issuppressed at the time of valve timing control, so as to maintain goodcombustion characteristics.

The present invention provides a variable valve timing apparatusprovided in an engine having a plurality of cylinder groups, including achanging mechanism, a reference position learning portion, a learningcompletion confirming portion, and a control start limiting portion. Thechanging mechanism is provided corresponding to each of the plurality ofcylinder groups, and each is configured to change, in the correspondingcylinder group, an opening/closing timing of at least one of an intakevalve and an exhaust valve. The reference position learning portion isprovided corresponding to each changing mechanism, and is configured togenerate an operation command for changing the opening/closing timing toa prescribed timing to the actuator of the corresponding changingmechanism when reference position learning is instructed and, when theopening/closing timing has reached the prescribed timing, to learn thereference of the opening/closing timing in response. The learningcompletion confirming portion confirms whether the reference positionlearning by the reference position learning portion has been completedin every changing mechanism or not. The control start limiting portioninhibits, in each changing mechanism, start of the opening/closingtiming control for changing the opening/closing timing following asuccessively set target value, until it is confirmed by the learningcompletion confirming portion that the reference position learning hasbeen completed in every changing mechanism.

Alternatively, the present invention provides a variable valve timingapparatus provided in an engine having a plurality of cylinder groups,including a changing mechanism, and a control unit for controlling theoperation of the changing mechanism. The changing mechanism is providedcorresponding to each of the plurality of cylinder groups, and each isconfigured to change, in the corresponding cylinder group, anopening/closing timing of at least one of an intake valve and an exhaustvalve. The control unit is configured to execute reference positionlearning independently for each changing mechanism, and furtherconfigured to generate an operation command for changing theopening/closing timing to a prescribed timing to the actuator of thecorresponding changing mechanism when the reference position learning isinstructed, when the opening/closing timing reaches the prescribedtiming, to learn the reference of the opening/closing timing inresponse. The control unit is further configured to confirm whether thereference position learning has been completed in every changingmechanism or not, and to inhibit, in each changing mechanism, start ofthe opening/closing timing control for changing the opening/closingtiming following a successively set target value, until it is confirmedthat the reference position learning has been completed in everychanging mechanism.

The present invention provides a method of controlling a variable valvetiming apparatus having, for every cylinder group, a changing mechanismfor changing a timing of opening/closing at least one of an intake valveand an exhaust valve provided in an engine, including a referenceposition learning step, a learning completion confirming step, and acontrol start limiting step. In the reference position learning step,which is executed corresponding to each changing mechanism, an operationcommand for changing the opening/closing timing to a prescribed timingis generated to the actuator of the corresponding changing mechanismwhen reference position learning is instructed, and when theopening/closing timing has reached the prescribed timing, reference ofthe opening/closing timing is learned in response. In the learningcompletion confirming step, whether the reference position learning ofthe reference position learning step has been completed in everychanging mechanism or not is confirmed. In the control start limitingstep, start of the opening/closing timing control for changing theopening/closing timing following a successively set target value isinhibited in each changing mechanism, until it is confirmed in thelearning completion confirming step that the reference position learninghas been completed in every changing mechanism.

According to the variable valve timing apparatus or the control methodthereof, until it is confirmed that the reference position learning hasbeen completed in the changing mechanism of every cylinder group (bank),starting of valve timing control following the target value can beinhibited in each bank. Therefore, variation in the amount of intake airto the cylinders among the banks can be suppressed at the time of valvetiming control, whereby satisfactory combustion characteristics can bemaintained.

Preferably, in the variable valve timing apparatus in accordance withthe present invention, each changing mechanism is configured to changethe opening/closing timing by an amount of change in accordance with anoperation amount of the actuator, and further configured such that thechange in opening/closing timing is mechanically limited at theprescribed timing, at least at the time of reference position learning.The reference position learning portion generates an actuator operationcommand in each changing mechanism so that the timing at which theopening/closing timing reaches the prescribed timing differs from onechanging mechanism to another, when the reference position learning isinstructed. Alternatively, the control unit generates an actuatoroperation command in each changing mechanism so that the timing at whichthe opening/closing timing reaches the prescribed timing differs fromone changing mechanism to another, when the reference position learningis instructed.

Preferably, in the method of controlling a variable valve timingapparatus in accordance with the present invention, each changingmechanism is configured to change the opening/closing timing by anamount corresponding to the operation amount of the actuator, andfurther configured such that the change in opening/closing timing ismechanically limited at the prescribed timing, at least at the time ofreference position learning. In the reference position learning step, anactuator operation command in each changing mechanism is generated sothat the timing at which the opening/closing timing reaches theprescribed timing differs from one changing mechanism to another, whenthe reference position learning is instructed.

According to the variable valve timing apparatus or the control methodthereof described above, in a configuration in which the change inopening/closing timing (valve timing) is mechanically limited at the endof the reference position learning and the power consumption by theactuator increases, the timing of executing the reference positionlearning is shifted among the cylinder groups (banks), so that increasein power load caused by concentration of power consumption can beavoided. It follows that the timing of completion of the referenceposition learning also comes to differ bank by bank. Therefore, start ofthe valve timing control is inhibited until it is confirmed thatreference position learning has been completed in every bank, wherebyvariation of the amount of air introduced to the cylinder among thebanks can be suppressed while the valve timing is controlled.

Preferably, the variable valve timing apparatus in accordance with thepresent invention further includes an increasing portion. The increasingportion increases the amount of air introduced to the engine when thereference position learning is executed while the engine is in an idleoperation, to be larger than when the reference position learning is notexecuted under the same conditions. Alternatively, the control unitgenerates a control command for increasing the amount of air introducedto the engine when the reference position learning is executed while theengine is in an idle operation, to be larger than when the referenceposition learning is not executed under the same conditions.

Preferably, in the method of controlling the variable valve timingapparatus in accordance with the present invention further includes anincreasing step. In the increasing step, when the reference positionlearning is executed while the engine is in idle operation, an amount ofair introduced to the engine is increased to be larger than when thereference position learning is not executed under the same conditions.

According to the variable valve timing apparatus or the control methodthereof described above, when the reference position learning isexecuted while the engine is in an idle operation, the amount of airintroduced to the engine can be increased relatively. Therefore, theopening/closing timing (valve timing) may be set to the prescribedtiming for the reference position learning without the necessity ofconsidering combustion characteristics, and hence, engine stall causedby deteriorated combustion characteristics can be prevented.

Further, in the variable valve timing apparatus or the control methodthereof in accordance with the present invention, in each changingmechanism, the prescribed timing is provided at a mechanical limitposition of the variable range of the opening/closing timing.

According to the variable valve timing apparatus or the control methodthereof described above, the reference position learning can be executedwithout adding any special mechanism, by using the limit position (suchas the phase of most retarded angle) of the variable range ofopening/closing timing (valve timing).

Alternatively, or more preferably, in the variable valve timingapparatus or the control method thereof in accordance with the presentinvention, the actuator is implemented by an electric motor, and thechanging mechanism is configured to change the opening/closing timing byan amount of change corresponding to the difference in rotation speed ofthe electric motor relative to the rotation speed of the camshaftdriving the valve of which opening/closing timing is to be changed.

According to the variable valve timing apparatus or the control methodthereof described above, in a configuration in which an electric motorserves as the actuator and the operation amount of the actuator isdifference in rotation speed of the electric motor relative to therotation speed of a camshaft of which rotation is stopped as the enginestops, variation of the amount of air introduced to cylinders among thecylinder groups (banks) can be suppressed at the time of valve timingcontrol, and satisfactory combustion characteristics can be maintained.

Therefore, a main advantage of the present invention is, in aconfiguration having a VVT mechanism provided in every one of aplurality of cylinder groups (banks), that the reference positionlearning can be executed appropriately to suppress variation of theamount of air introduced to the cylinders among the cylinder groups(banks) at the time of valve timing control and to maintain satisfactorycombustion characteristics.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing a configuration of an engine of avehicle on which the variable valve timing apparatus in accordance withan embodiment of the present invention is mounted.

FIG. 2 shows a map defining the phase of an intake camshaft.

FIG. 3 is a cross section showing an intake VVT mechanism.

FIG. 4 is a cross section along A-A in FIG. 3.

FIG. 5 is a (first) cross section along B-B in FIG. 3.

FIG. 6 is a (second) cross section along B-B in FIG. 3.

FIG. 7 is a cross section along C-C in FIG. 3.

FIG. 8 is a cross section along D-D in FIG. 3.

FIG. 9 shows the reduction gear ratio of the intake VVT mechanism as awhole.

FIG. 10 shows a relation between the phase of a guide plate relative toa sprocket and the phase of an intake camshaft.

FIG. 11 is a schematic block diagram illustrating a control structure ofintake valve phase by the variable valve timing apparatus in accordancewith the present embodiment.

FIG. 12 is a block diagram illustrating rotation speed control of anelectric motor as the actuator of the variable valve timing apparatus inaccordance with the present embodiment.

FIG. 13 illustrates speed control of the electric motor.

FIG. 14 is a flowchart representing the reference position learning inthe variable valve timing apparatus in accordance with an embodiment ofthe present invention.

FIG. 15 is a diagram of waveforms at the time of reference positionlearning shown in FIG. 14.

FIG. 16 is a flowchart representing determination as to whether thevalve timing control may be started or not, in the variable valve timingapparatus in accordance with an embodiment of the present invention.

FIG. 17 is a diagram of waveforms representing the timing of executingthe reference position learning corresponding to the engine start.

FIG. 18 is a flowchart representing the manner when the referenceposition learning is executed during an idle operation.

BEST MODES FOR CARRYING OUT THE INVENTION

With reference to the drawings, embodiments of the present inventionwill be hereinafter described. In the following description, likecomponents are denoted by like reference characters. Their names andfunctions are also the same. Therefore, detailed description thereofwill not be repeated.

Referring to FIG. 1, a description is given of an engine of a vehicle onwhich a variable valve timing apparatus is mounted, according to anembodiment of the present invention.

An engine 1000 is a V-type 8-cylinder engine having a first bank 1010and a second bank 1012 each including a group of four cylinders. Here,application of the present invention is not limited to any engine type,and the variable valve timing apparatus that will be described in thefollowing is applicable to an engine of the type different from theV-type 8 cylinder engine.

Into engine 1000, air is sucked from an air cleaner 1020. The quantityof sucked air is adjusted by a throttle valve 1030. Throttle valve 1030is an electronic throttle valve driven by a motor.

The air is supplied through an intake manifold 1032 into a cylinder1040. The air is mixed with fuel in cylinder 1040 (combustion chamber).Into cylinder 1040, the fuel is directly injected from an injector 1050.In other words, injection holes of injector 1050 are provided withincylinder 1040.

The fuel is injected in the intake stroke. The fuel injection timing isnot limited to the intake stroke. Further, in the present embodiment,engine 1000 is described as a direct-injection engine having injectionholes of injector 1050 that are disposed within cylinder 1040. However,in addition to direct-injection (in-cylinder) injector 1050, a portinjector may be provided. Moreover, only the port injector may beprovided.

The air-fuel mixture in cylinder 1040 is ignited by a spark plug 1060and accordingly burned. The air-fuel mixture after burned, namelyexhaust gas, is cleaned by a three-way catalyst 1070 and thereafterdischarged to the outside of the vehicle. The air-fuel mixture is burnedto press down a piston 1080 and thereby to rotate a crankshaft 1090.

At the top of cylinder 1040, an intake valve 1100 and an exhaust valve1110 are provided. Intake valve 1100 is driven by an intake camshaft1120. Exhaust valve 1110 is driven by an exhaust camshaft 1130. Intakecamshaft 1120 and exhaust camshaft 1130 are coupled by such parts as achain and gears to be rotated at the same rotation speed (one-half therotation speed of crankshaft 1090). The rotation speed of a rotatingbody such as a shaft is generally represented by the number of rotationsper unit time (typically, number of rotations per minute: rpm).

Intake valve 1100 has its phase (opening/closing timing) controlled byan intake VVT mechanism 2000 provided to intake camshaft 1120. Exhaustvalve 1110 has its phase (opening/closing timing) controlled by anexhaust VVT mechanism 3000 provided to exhaust camshaft 1130.

In the present embodiment, intake camshaft 1120 and exhaust camshaft1130 are rotated by the VVT mechanisms to control respective phases ofintake valve 1100 and exhaust valve 1110. Here, the phase control methodis not limited to the one described above. As shown in the figure, theVVT mechanism is provided bank by bank.

Intake VVT mechanism 2000 is operated by an electric motor 2060 (shownin FIG. 3). Electric motor 2060 is controlled by an Electronic ControlUnit (ECU) 4000. The current and voltage of electric motor 2060 aredetected by an ammeter (not shown) and a voltmeter (not shown) and themeasurements are input to ECU 4000.

Exhaust VVT mechanism 3000 is hydraulically operated. Here, intake VVTmechanism 2000 may be hydraulically operated while exhaust VVT mechanism3000 may be operated by an electric motor.

To ECU 4000, signals indicating the rotation speed and the crank angleof crankshaft 1090 are input from a crank angle sensor 5000. Further, toECU 4000, signals indicating respective phases of intake camshaft 1120and exhaust camshaft 1130 (phase: the camshaft position in therotational direction) are input from a cam position sensor 5010.

Furthermore, to ECU 4000, a signal indicating the water temperature(coolant temperature) of engine 1000 from a coolant temperature sensor5020 as well as a signal indicating the quantity of intake air (quantityof air taken or sucked into engine 1000) of engine 1000 from an airflowmeter 5030 are input.

Based on these signals input from the sensors as well as a map and aprogram stored in a memory (not shown), ECU 4000 controls the throttleopening position, the ignition timing, the fuel injection timing, thequantity of injected fuel, the phase of intake valve 1100 and the phaseof exhaust valve 1110, for example, so that engine 1000 is operated in adesired operating state.

In the present embodiment, ECU 4000 determines the phase of intake valve1100 based on the map as shown in FIG. 2 that uses the engine speed NEand the intake air quantity KL as parameters. A plurality of maps forrespective coolant temperatures are stored for determining the phase ofintake valve 1100.

In the following, a further description is given of intake VVT mechanism2000. Here, exhaust VVT mechanism 3000 may have the same configurationas that of intake VVT mechanism 2000 as described below, or each ofintake VVT mechanism 2000 and exhaust VVT mechanism 3000 may have thesame configuration as that of intake VVT mechanism 2000 as describedbelow.

As shown in FIG. 3, intake VVT mechanism 2000 includes a sprocket 2010,a cam plate 2020, a link mechanism 2030, a guide plate 2040, reductiongears 2050, and electric motor 2060.

Sprocket 2010 is coupled via a chain or the like to crankshaft 1090. Therotation speed of sprocket 2010 is half the rotation speed of crankshaft1090, as in the case of intake camshaft 1120 and exhaust camshaft 1130.Intake camshaft 1120 is provided concentrically with the rotational axisof sprocket 2010 and rotatable relative to sprocket 2010.

Cam plate 2020 is coupled to intake camshaft 1120 with a pin (1) 2070.Cam plate 2020 rotates, in sprocket 2010, together with intake camshaft1120. Here, cam plate 2020 and intake camshaft 1120 may be integratedinto one unit.

Link mechanism 2030 is comprised of an arm (1) 2031 and an arm (2) 2032.As shown in FIG. 4, which is a cross section along A-A in FIG. 3, a pairof arms (1) 2031 is provided within sprocket 2010 so that the arms arepoint symmetric to each other with respect to the rotational axis ofintake camshaft 1120. Each arm (1) 2031 is coupled to sprocket 2010 sothat the arm can swing about a pin (2) 2072.

As shown in FIG. 5, which is a cross section along B-B in FIG. 3, and asshown in FIG. 6 showing the state where the phase of intake valve 1100is advanced with respect to the state in FIG. 5, arms (1) 2031 and camplate 2020 are coupled by arms (2) 2032.

Arm (2) 2032 is supported such that the arm can swing about a pin (3)2074 and with respect to arm (1) 2031. Further, arm (2) 2032 issupported such that the arm can swing about a pin (4) 2076 and withrespect to cam plate 2020.

A pair of link mechanisms 2030 causes intake camshaft 1120 to rotaterelative to sprocket 2010 and thereby changes the phase of intake valve1100. Thus, even if one of the paired link mechanisms 2030 should bedamaged or broken, the other link mechanism can be used to change thephase of intake valve 1100.

Referring back to FIG. 3, at a surface of each link mechanism 2030 (arm(2) 2032) that is a surface facing guide plate 2040, a control pin 2034is provided. Control pin 2034 is provided concentrically with pin (3)2074. Each control pin 2034 slides in a guide groove 2042 provided inguide plate 2040.

Each control pin 2034 slides in guide groove 2042 of guide plate 2040,to be shifted in the radial direction. The radial shift of each controlpin 2034 causes intake camshaft 1120 to rotate relative to sprocket2010.

As shown in FIG. 7, which is a cross section along C-C in FIG. 3, guidegroove 2042 is formed in the spiral shape so that rotation of guideplate 2040 causes each control pin 2034 to shift in the radialdirection. Here, the shape of guide groove 2042 is not limited to this.

As control pin 2034 is shifted further in the radial direction from theaxial center of guide plate 2040, the phase of intake valve 1100 isretarded to a greater extent. In other words, the amount of change ofthe phase has a value corresponding to the operation amount of linkmechanism 2030 generated by the radial shift of control pin 2034.Alternatively, the phase of intake valve 1100 may be advanced to agreater extent as control pin 2034 is shifted further in the radialdirection from the axial center of guide plate 2040.

As shown in FIG. 7, when control pin 2034 abuts on an end of guidegroove 2042, the operation of link mechanism 2030 is restrained.Therefore, the phase in which control pin 2034 abuts on an end of guidegroove 2042 is the phase of the most retarded angle or the most advancedangle.

Referring back to FIG. 3, in guide plate 2040, a plurality of depressedportions 2044 are provided in its surface facing reduction gears 2050,for coupling guide plate 2040 and reduction gears 2050 to each other.

Reduction gears 2050 are comprised of an outer teeth gear 2052 and aninner teeth gear 2054. Outer teeth gear 2052 is fixed with respect tosprocket 2010 so that the gear rotates together with sprocket 2010.

Inner teeth gear 2054 has a plurality of protruded portions 2056 thereonthat are received in depressed portions 2044 of guide plate 2040. Innerteeth gear 2054 is supported rotatably about an eccentric axis 2066 of acoupling 2062 formed eccentrically with respect to an axial center 2064of an output shaft of electric motor 2060.

FIG. 8 shows a cross section along D-D in FIG. 3. Inner teeth gear 2054is provided such that a part of the teeth thereof meshes with outerteeth gear 2052. When the rotation speed of the output shaft of electricmotor 2060 is identical to the rotation speed of sprocket 2010, coupling2062 and inner teeth gear 2054 rotate at the same rotation speed as thatof outer teeth gear 2052 (sprocket 2010). In this case, guide plate 2040rotates at the same rotation speed as that of sprocket 2010 andaccordingly the phase of intake valve 1100 is maintained.

When electric motor 2060 causes coupling 2062 to rotate about axialcenter 2064 and relative to outer teeth gear 2052, inner teeth gear 2054as a whole accordingly revolves about axial center 2064 while innerteeth gear 2054 rotates about eccentric axis 2066. The rotational motionof inner teeth gear 2054 causes guide plate 2040 to rotate relative tosprocket 2010 and thus the phase of intake valve 1100 is changed.

The phase of intake valve 1100 is changed by reduction of the rotationspeed of relative rotation between the output shaft of electric motor2060 and sprocket 2010 (operation amount of electric motor 2060) byreduction gears 2050, guide plate 2040 and link mechanism 2030. Here,the rotation speed of relative rotation between the output shaft ofelectric motor 2060 and sprocket 2010 may be increased to change thephase of intake valve 1100. On the output shaft of electric motor 2060,a motor rotation angle sensor 5050 is provided, which outputs a signalindicating an angle of rotation (position of the output shaft in therotating direction) of the output shaft. Motor rotation angle sensor5050 is generally configured to generate a pulse signal every time theoutput shaft of electric motor rotates by a prescribed angle. Based onthe output of motor rotation angle sensor 5050, the rotation speed ofthe output shaft of electric motor 2060 (hereinafter also simplyreferred to as rotation speed of electric motor 2060) can be detected.

As shown in FIG. 9, the reduction gear ratio R(θ) of intake VVTmechanism 2000 as a whole, that is, the ratio of rotation speed ofrelative rotation between the output shaft of electric motor 2060 andsprocket 2010 to the amount of phase-change, may have a value accordingto the phase of intake valve 1100. In the present embodiment, as thereduction gear ratio R(θ) is higher, the amount of phase-change withrespect to the rotation speed of relative rotation between the outputshaft of electric motor 2060 and sprocket 2010 is smaller.

In the case where the phase of intake valve 1100 is in a first regionfrom the most retarded angle to CA (1), the reduction gear ratio ofintake VVT mechanism 2000 as a whole is R (1). In the case where thephase of intake valve 1100 is in a second region from CA (2) (CA (2) isadvanced with respect to CA (1)) to the most advanced angle, thereduction gear ratio of intake VVT mechanism 2000 as a whole is R (2) (R(1)>R (2)).

In the case where the phase of intake valve 1100 is in a third regionfrom CA (1) to CA (2), the reduction gear ratio of intake VVT mechanism2000 as a whole changes at a predetermined rate of change ((R (2)−R(1))/(CA (2)−CA (1)).

Based on the configuration as described above, intake VVT mechanism 2000of the variable valve timing apparatus of the present embodimentfunctions as described below.

When the phase of intake valve 1100 (intake camshaft 1120) is to beadvanced, electric motor 2060 is operated to rotate guide plate 2040relative to sprocket 2010, thereby advancing the phase of intake valve1100 as shown in FIG. 10.

When the phase of intake valve 1100 is in the first region between themost retarded angle and CA (1), the rotation speed of relative rotationbetween the output shaft of electric motor 2060 and sprocket 2010 isreduced at reduction gear ratio R (1) and the phase of intake valve 1100is advanced.

In the case where the phase of intake valve 1100 is in the second regionbetween CA (2) and the most advanced angle, the rotation speed ofrelative rotation between the output shaft of electric motor 2060 andsprocket 2010 is reduced at reduction gear ratio R (2) and the phase ofintake valve 1100 is advanced.

When the phase of intake valve 1100 is to be retarded, the output shaftof electric motor 2060 is rotated relative to sprocket 2010 in thedirection opposite to the direction when the phase thereof is to beadvanced. As in the case of advancing the phase, when the phase is to beretarded and the phase of intake valve 1100 is in the first regionbetween the most retarded angle and CA (1), the rotation speed ofrelative rotation between the output shaft of electric motor 2060 andsprocket 2010 is reduced at reduction gear ratio R (1) and the phase isretarded. Further, when the phase of intake valve 1100 is in the secondregion between CA (2) and the most advanced angle, the rotation speed ofrelative rotation between the output shaft of electric motor 2060 andsprocket 2010 is reduced at reduction gear ratio R (2) and the phase isretarded.

Accordingly, as long as the direction of the relative rotation betweenthe output shaft of electric motor 2060 and sprocket 2010 is the same,the phase of intake valve 1100 can be advanced or retarded for both ofthe first region between the most retarded angle and CA (1) and thesecond region between CA (2) and the most advanced angle. Here, for thesecond region between CA (2) and the most advanced angle, the phase canbe more advanced or more retarded. Thus, the phase can be changed over awide range.

Further, since the reduction gear ratio is high for the first regionbetween the most retarded angle and CA (1), a large torque is necessary,for rotating the output shaft of electric motor 2060 by a torque actingon intake camshaft 1120 as engine 1000 operates. Therefore, even ifelectric motor 2060 generates no torque as in the case where electricmotor 2060 is stopped, rotation of the output shaft of electric motor2060 caused by the torque acting on intake camshaft 1120 can beprevented. Therefore, a change of the actual phase from a phasedetermined under control can be restrained.

As described above, in intake VVT mechanism 2000, as there is thereduction gear ratio R(θ), unintended change in phase is less likelywhen power supply to electric motor 2060 as the actuator is stopped.This effect is particularly well achieved in the first region thatcovers the phase of the most retarded angle.

When the phase of intake valve 1100 is in the third region between CA(1) and CA (2), the rotation speed of relative rotation between theoutput shaft of electric motor 2060 and sprocket 2010 is reduced at areduction gear ratio that changes at a predetermined rate of change,which may result in advance or retard in phase of intake valve 1100.

Accordingly, when the phase changes from the first region to the secondregion or from the second region to the first region, the amount ofchange of the phase with respect to the rotation speed of relativerotation between the output shaft of electric motor 2060 and sprocket2010 can be increased or decreased gradually. In this way, a suddenstepwise change of the amount of change in phase can be restrained, tothereby restrain a sudden change in phase. Accordingly, phasecontrollability can be improved.

As discussed above, in the intake VVT mechanism for the variable valvetiming apparatus in the present embodiment, when the phase of the intakevalve is in the region from the most retarded angle to CA (1), reductiongear ratio of intake VVT mechanism 2000 as a whole is R (1). When thephase of the intake valve is in the region from CA (2) to the mostadvanced angle, the reduction gear ratio of intake VVT mechanism 2000 asa whole is R (2), which is lower than R (1). Thus, as long as therotational direction of the output shaft of the electric motor is thesame, the phase of the intake valve can be advanced or retarded for bothof the regions, namely the first region between the most retarded angleand CA (1) and the second region between CA (2) and the most advancedangle. Here, for the second region between CA (2) and the most advancedangle, the phase can be advanced or retarded to a greater extent.Therefore, the phase can be changed over a wide range. Further, for thefirst region between the most retarded angle and CA (1), the reductiongear ratio is high and therefore, it is possible to prevent rotation ofthe output shaft of the electric motor by the torque acting on theintake camshaft as the engine is operated. Thus, a change of the actualphase from a phase determined under control can be restrained.Accordingly, the phase can be changed over a wide range and the phasecan be controlled accurately.

Next, the structure for controlling the phase of intake valve 1100(hereinafter also simply referred to as the intake valve phase) will bedescribed in detail.

Referring to FIG. 11, as already described with reference to FIG. 1,engine 1000 is configured such that power from crankshaft 1090 istransmitted to intake camshaft 1120 and exhaust camshaft 1130 throughsprockets 2010 and 2012, respectively, by means of a timing chain 1200(or a timing belt). Further, on the outer circumferential side of intakecamshaft 1120, a cam position sensor 5010 is attached, for outputting acam angle signal Piv, at every prescribed cam angle. On the outercircumferential side of crankshaft 1090, a crank angle sensor 5000 isattached, for outputting a crank angle signal Pca, at every prescribedcrank angle. Further, on a rotor (not shown) of electric motor 2060, amotor rotation angle sensor 5050 is attached, for outputting a motorrotation angle signal Pmt, at every prescribed rotation angle. The camangle signal Piv, crank angle signal Pca and motor rotation angle signalPmt are input to ECU 4000.

Further, based on the outputs of sensors detecting the state of engine1000 and on operation conditions (pedal operation of the driver, currentvehicle speed and the like), ECU 4000 controls the operation of engine1000 so that required output of engine 1000 can be attained. As a partof engine control, ECU 4000 sets phase target values of intake valve1100 and exhaust valve 1110, based on the map shown in FIG. 2.

Further, ECU 4000 generates a rotation speed command value Nmref ofelectric motor 2060 as the actuator of intake VVT mechanism 2000 suchthat the phase of intake valve 1100 reaches the target phase. Therotation speed command Nmref is determined corresponding to the rotationspeed of output shaft of electric motor 2060 relative to sprocket 2010(intake camshaft 1120), as will be described later. The difference inrotation speed of electric motor 2060 relative to intake camshaft 1120corresponds to the operation amount of actuator. Motor EDU (ElectronicDrive Unit) 4100 controls the rotation speed of electric motor 2060, inaccordance with the rotation speed command Nmref from ECU 4000.

FIG. 12 is a block diagram illustrating rotation speed control ofelectric motor 2060 as the actuator of intake VVT mechanism 2000 inaccordance with the present embodiment.

Referring to FIG. 12, an actuator operation amount setting portion 6000includes a valve phase detecting portion 6010, a camshaft phase-changeamount calculating portion 6020, a relative rotation speed settingportion 6030, a camshaft rotation speed detecting portion 6040, arotation speed command value generating portion 6050, and a switchingportion 6150. Actuator operation amount setting portion 6000 executesvalve timing control by which the phase of intake valve 1100 is changedfollowing a target value IV(θ)r that is set successively in accordancewith the map of FIG. 2.

Further, a learning control portion 6100 is provided, for learning thereference position of the intake valve phase. The operations of actuatoroperation amount setting portion 6000 and learning control portion 6100are realized by executing a control process in accordance with aprescribed program stored in advance in ECU 4000 at every prescribedcontrol period.

Valve phase detecting portion 6010 calculates the currently detectedintake valve phase IV(θ) (hereinafter also denoted as phase detectionvalue IV(θ)), based on crank angle signal Pca from crank angle sensor5000, cam angle signal Piv from cam position sensor 5010 and motorrotation angle signal Pmt from rotation angle sensor 5050 of electricmotor 2060.

Valve phase detecting portion 6010 may calculate the phase detectionvalue IV(θ) based on crank angle signal Pca and on cam angle signal Piv.By way of example, at the time when cam angle signal Piv is generated,time difference of cam angle signal Piv from the generation of crankangle signal Pca is converted to rotation phase difference betweencrankshaft 1090 and intake camshaft 1120, whereby the current phasedetection value IV(θ) of intake camshaft 1120 may be calculated (firstphase calculating method).

Alternatively, in intake VVT mechanism 2000 in accordance with anembodiment of the present invention, it is possible to accurately tracethe phase-change amount of intake valve based on the operation amount(relative rotation speed ΔNm) of electric motor 2060 as the actuator.Specifically, based on the outputs of various sensors, the actualrelative rotation speed ΔNm is calculated, and by an operation inaccordance with expression (1) described later based on the calculatedactual relative rotation speed ΔNm, the amount of change dIV(θ) of theintake valve phase per unit time (control period) can be calculated.Therefore, by accumulating the amount of phase-change dIV(θ), valvephase detecting portion 6010 may calculate the current phase detectionvalue IV(θ) of intake camshaft 1120 successively (second phasecalculating method). Valve phase detecting portion 6010 may calculatethe phase detection value IV(θ) by appropriately using the first andsecond phase calculating methods, in consideration of stability inengine speed or computational load.

Camshaft phase-change amount calculating portion 6020 has a calculatingportion 6022 and a necessary phase-change amount calculating portion6025. Calculating portion 6022 calculates deviation ΔIV(θ) in phase,(ΔIV(θ)=IV(θ)−IV(θ)r), from the target phase IV(θ)r. Necessaryphase-change amount calculating portion 6025 calculates the necessaryamount of change Δθ of intake camshaft 1120 of this control period, inaccordance with the deviation ΔIV(θ) calculated by calculating portion6022.

By way of example, the maximum value Δθmax of phase-change amount Δθ ina single control period is set in advance, and necessary phase-changeamount calculating portion 6025 determines the phase-change amount Δθ inaccordance with the phase deviation ΔIV(θ) within the range up to themaximum value Δθmax. Here, the maximum value Δθmax may be a prescribedfixed value, or it may be variably set by necessary phase-change amountcalculating portion 6025 in accordance with the state of operation(rotation speed, amount of intake air and the like) of engine 1000 orthe magnitude of phase deviation ΔIV(θ).

Relative rotation speed setting portion 6030 calculates relativerotation speed ΔNm of the output shaft of electric motor 2060 relativeto the rotation speed of sprocket 2010 (intake camshaft 1120), necessaryto generate the phase-change amount Δθ calculated by necessaryphase-change amount calculating portion 6025. By way of example, therelative rotation speed ΔNm is set to a positive value (ΔNm>0) when theintake valve phase is to be advanced, set to a negative value (ΔNm<0)when the intake valve phase is to be retarded, and set to approximatelyzero (ΔNm=0) when the current intake valve phase is to be maintained(Δθ=0).

Here, the relation between the phase-change amount Δθ per unit time ΔTcorresponding to the control period and the relative rotation speed ΔNmis represented by the following expression (1). In expression (1), R(θ)represents reduction gear ratio that changes in accordance with theintake valve phase, shown in FIG. 9.

Δθ∝ΔNm.360°.(1/R(θ)).ΔT  (1)

Therefore, relative rotation speed setting portion 6030 may calculatethe relative rotation speed ΔNm of electric motor 2060 for generatingthe camshaft phase-change amount Δθ required in control period ΔT, inaccordance with an operation of expression (1).

Camshaft rotation speed detecting portion 6040 calculates the rotationspeed of sprocket 2010, that is, the actual rotation speed IVN of intakecamshaft 1120 as one-half the rotation speed of crankshaft 1090.Camshaft rotation speed detecting portion 6040 may be configured tocalculate the actual rotation speed IVN of intake camshaft 1120 based onthe cam angle signal Piv from cam position sensor 5010. Generally,however, the number of cam angle signals output per one rotation ofintake camshaft 1120 is smaller than the number of crank angle signalsoutput per one rotation of crankshaft 1090. Therefore, by detecting thecamshaft rotation speed IVN based on the rotation speed of crankshaft1090, detection accuracy can be improved.

Switching portion 6150 is arranged between rotation speed command valuegenerating portion 6050 and relative rotation speed setting portion 6030and learning control portion 6100. Switching portion 6150 inputs therelative rotation speed ΔNm set by relative rotation speed settingportion 6030 to rotation speed command value generating portion 6050except when the reference position learning by learning control portion6100 is being executed. The reference position learning in accordancewith the present embodiment will be described in detail later.

Rotation speed command value generating portion 6050 adds the actualrotation speed IVN of intake camshaft 1120 detected by camshaft rotationspeed detecting portion 6040 and the relative rotation speed ΔNm inputfrom switching portion 6150 to generate rotation speed command valueNmref of electric motor 2060. Therefore, during operations including thenormal operation, other than at the time of reference position learning,the rotation speed command value Nmref of electric motor 2060 isgenerated based on the relative rotation speed ΔNm set by relativerotation speed setting portion 6030. At the time of reference positionlearning, the rotation speed command value Nmref of electric motor 2060is generated based on the relative rotation speed ΔNm0 set by learningcontrol portion 6100. The rotation speed command value Nmref generatedby rotation speed command value generating portion 6050 is transmittedto motor EDU 4100.

Motor EDU 4100 is connected to a power source 4200 through a relaycircuit 4250. On/off of relay circuit 4250 is controlled by a controlsignal SRL. Generally, power source 4200 is formed by a secondarybattery that can be charged when the engine operates. Therefore, byturning off the relay circuit, power supply to electric motor 2060 canbe stopped.

Motor EDU 4100 executes rotation speed control such that the rotationspeed of electric motor 2060 matches the rotation speed command valueNmref. By way of example, motor EDU 4100 controls switching of a powersemiconductor device (such as a transistor) such that the power suppliedto electric motor 2060 (as represented by motor current Imt) from apower source 4200 is controlled in accordance with deviation in rotationspeed (Nref−Nm) of actual rotation speed Nm of electric motor 2060 fromthe rotation speed command value Nmref. Specifically, the duty ratio ofswitching operation of such power semiconductor device is controlled.

Particularly, in order to improve motor controllability, motor EDU 4100controls duty ratio DTY as the amount of adjustment in rotation speedcontrol in accordance with the following equation (2).

DTY=DTY(ST)+DTY(FB)  (2)

In Equation (2), DTY(FB) is a feedback term based on the deviation inrotation speed mentioned above and a control operation (typically,general P control, PI control or the like) with a prescribed controlgain.

In Equation (2), DTY(ST) is a preset term set based on the rotationspeed command value Nmref of electric motor 2060 and the set relativerotation speed ΔNm.

Referring to FIG. 13, duty ratio characteristic 6060 corresponding tothe motor current value required when relative rotation speed ΔNm=0,that is, when electric motor 2060 is to be rotated at the same rotationspeed as that of sprocket 2060 with respect to rotation speed commandvalue Nmref (ΔNm=0), is set in advance as a table. Then, DTY(ST) inEquation (2) is set by relative addition/subtraction of a current valuecorresponding to the relative rotation speed ΔNm to/from the referencevalue in accordance with duty ratio characteristic 6060. By suchrotation speed control that the power supply to electric motor 2060 iscontrolled by the combination of preset term and feedback term, motorEDU 4100 allows the rotation speed of electric motor 2060 to quicklyfollow any change in rotation speed command value Nmref, as comparedwith simple feedback control, that is, the rotation speed control simplyby the term DTY(FB) of Equation (2).

(Reference Position Learning in Accordance With an Embodiment of thePresent Invention)

In order to improve accuracy in detecting the phase of intake camshaft1120, intake VVT mechanism 2000 performs reference position learning ofthe intake valve phase, using learning control portion 6100, whenprescribed conditions instructing learning are satisfied. In the presentembodiment of the invention, the reference position learning is done ina region where the reduction gear ratio R(θ) is large. Specifically, thereference position learning is done by causing the intake valve phase toreach the most retarded angle.

Referring to FIG. 12, learning control portion 6100 sets the relativerotation speed ΔNm0 of electric motor 2060 as the actuator operationamount for performing reference position learning, in response to alearning instructing signal that is turned “on” when prescribedconditions instructing learning are satisfied. At the time of referenceposition learning, switching portion 6150 inputs the output of learningcontrol portion 6100 to rotation speed command value generating portion6050, and therefore, based on the relative rotation speed ΔNm0 set bylearning control portion 6100, the rotation speed command value Nmref ofelectric motor 2060 is generated.

During reference position learning in which electric motor 2060 operatesin accordance with the relative rotation speed ΔNm0, learning controlportion 6100 determines whether the intake valve phase has reached themost retarded angle (for example, 0°) as the reference phase, based onthe phase detection value IV(θ) detected by valve phase detectingportion 6010.

When it is detected that the intake valve phase has reached thereference phase, learning control portion 6100 ends the learningoperation, and sets the phase detection value IV(θ) at that time asphase learning value θ1n.

The phase learning value θ1n calculated in this manner is reflected onthe detection of the intake valve phase in the valve timing controlthereafter. By way of example, valve timing is controlled regarding therelative difference between the phase detecting value IV(θ) obtained byvalve phase detecting portion 6010 and the phase learning value θ1ndescribed above as the difference between the actual intake valve phaseand the reference phase (that is, 0°) at the time of reference positionlearning. Specifically, the phase learning value θ1n is reflected on thecalculation of phase deviation ΔIV(θ) at calculating portion 6022.

FIG. 14 shows a flowchart representing the reference position learningin accordance with the embodiment of the present invention, and FIG. 15shows operation waveforms at the time of reference position learning.The reference position learning routine in accordance with the flowchartof FIG. 14 is executed at a prescribed period by ECU 4000.

Referring to FIG. 14, at step S100, ECU 4000 determines whetherprescribed learning execution conditions are satisfied or not. Asdescribed with reference to FIG. 9, in intake VVT mechanism 2000 inaccordance with the present embodiment, possibility of unintended phasechange is low when power supply to electric motor 2060 as the actuatoris stopped, because of the reduction gear ratio R(θ). Therefore, bystoring the phase detection values IV(θ), which are successivelydetected in ECU 4000, in a memory area (such as an SRAM: Static RandomAccess Memory) that retains the stored contents even when the ignitionswitch is off (when the operation is stopped), it becomes unnecessary toperform the reference position learning every time the engine isstarted. When such an arrangement is adopted, the conditions forexecuting learning of step S100 may be satisfied when the contentsstored in the memory are cleared, for example, at the time of batterychange or the like.

Alternatively, in order to improve accuracy in detecting the intakevalve phase, the conditions for executing learning of step S100 may besatisfied every time the engine is started.

When the conditions for executing learning are not satisfied (NO at stepS100), ECU 4000 ends the process, as the reference position learning isnot instructed.

On the other hand, when the conditions for executing learning aresatisfied (YES at step S100), ECU 4000 turns “on” the learninginstructing signal input to learning control portion 6100 (FIG. 12), andexecutes the reference position learning through the steps followingstep S110.

At step S110, ECU 4000 sets the relative rotation speed ΔNm0 of electricmotor 2060, as the actuator operation amount for performing thereference position learning. The relative rotation speed ΔNm0 is set toa value for changing the intake valve phase to the most retarded angle(0°) as the reference phase. Specifically, in the present embodiment,the relative rotation speed ΔNm0 is set to a prescribed negative value.This corresponds to the operation of learning control portion 6100 inresponse to turning “on” of the learning instructing signal shown inFIG. 12.

Referring to FIG. 15, when the conditions for executing learning aresatisfied and the learning instructing signal is turned “on” at timepoint t0, electric motor 2060 operates in accordance with relativerotation speed command value ΔNm0 (<0), whereby the phase detectionvalue IV(θ) is retarded at a constant rate.

When the actual intake valve phase attains to the most retarded angle(0°) at time point t1, operation of link mechanism 2030 is locked, andthe amount of change in intake valve phase becomes approximately zero.At this time, the relative rotation speed of electric motor 2060 alsobecomes approximately zero.

When there is an offset error in the phase detection value IV(θ), theactual intake valve phase reaches the most retarded angle beforeIV(θ)=0, and the relative rotation speed of electric motor 2060 attainsto zero and the change in phase detection value IV(θ) stops. Therefore,whether the actual intake valve phase has reached the most retardedangle as the reference phase or not can be detected based on the amountof change in phase detecting value IV(θ), or on the amount of changedIV(θ) based on the actual relative rotation speed ΔNm, that is, thevalue dIV(θ) attaining to dIV(θ)≈0.

In response, the reference position learning is completed, and alearning complete flag is turned “on”. The phase detection value IV(θ)at this time is stored as the phase learning value θ1n, and reflected onthe valve timing control thereafter.

Again referring to FIG. 14, in order to realize the operation after timepoint t0 of FIG. 15, ECU 4000 executes following steps S120 to S160.

At step S120, ECU 4000 detects a change in intake valve phase by theoperation of electric motor 2060 in accordance with relative rotationspeed ΔNm0. This corresponds to calculation of phase detection valueIV(θ) by valve phase detecting portion 6010.

Further, at step S130, ECU 4000 calculates the amount of phase-changebased on the detection of intake valve phase at step S120. This isequivalent to calculation of phase-change amount dIV(θ), as describedabove.

At step S140, ECU 4000 compares the phase-change amount calculated atstep S130 with a determination value θ0. The determination value θ0 isset to a prescribed value near zero, so as to enable detection that thephase-change amount reached approximately zero.

When the phase-change amount≧θ0 (NO at step S140), ECU 4000 determinesthat the actual intake valve phase is not yet reached the referencephase (most retarded angle), and at step S150, continues power supply toelectric motor 2060, thereby to continue reference position learning.Specifically, between time points t0 and t1 of FIG. 15, step S150 isexecuted.

When the phase-change amount<θ0 (YES at step S140), ECU 4000 determinesthat the actual intake valve phase has reached the reference phase (mostretarded angle), completes reference position learning, and obtains thephase learning value θ1n. Specifically, at time point t1 of FIG. 15,step S160 is executed.

Further, at the completion of reference position learning at which theamount of change in intake valve phase is approximately zero, it is in alocked state, and therefore, increase in motor current is expected.Therefore, when the timing at which the motor current increases overlapsamong banks at the end of learning, power load increasesinstantaneously. Therefore, it is preferred that the reference positionlearning is performed with the timing of completion of the referenceposition learning (time point t1 of FIG. 15) shifted among the pluralityof banks. By way of example, the increase in power load mentioned abovecan be avoided by shifting the timing of starting reference positionlearning (time point t0 of FIG. 15) among the plurality of banks.

In the variable valve timing apparatus in accordance with the presentembodiment, in an engine configured with a plurality of banks, adetermination as to whether the valve timing control may be started ornot such as shown in FIG. 16 is executed, in order to prevent variationin engine speed caused by deviation in valve timing setting among thebanks.

Referring to FIG. 16, at the end of reference position learning, ECU4000 determines whether the valve timing control may be started or notin each bank, through steps S170 to S195 as described in the following.

At step S170, ECU 4000 confirms whether the learning completion flag is“on” and the learning has been completed, in the corresponding bank.Further, when the learning has been completed in the corresponding bank(YES at step S170), at step S180, ECU 4000 confirms whether the learningcompletion flag is “on” and the learning has been completed in otherbanks.

When the reference position learning has been completed in thecorresponding and other banks, that is, when the reference positionlearning has been completed in all banks (YES at both steps S170 andS180), at step S190, ECU 4000 permits intake VVT mechanism 2000 of thecorresponding bank to start valve timing control. As a result, by thevalve timing control reflecting the result of reference positionlearning, the phase of intake valve 1100 is changed following the targetvalue IV(θ)r successively set in accordance with the map of FIG. 2.

When it is NO either at step S170 or S180, that is, when the referenceposition learning is completed in any of the banks, at step S195, ECU4000 inhibits start of the valve timing control by intake VVT mechanism2000 of the corresponding bank. Specifically, until it is confirmed thatthe reference position learning is completed in every bank, start ofvalve timing control is limited in each bank, even if the referenceposition learning has been completed in that bank.

Because of such a configuration, in the variable valve timing apparatusin accordance with the present embodiment, it becomes possible to havevalve timing settings matched among the plurality of banks, to preventvariation in the amount of air introduced to the cylinders, among thebanks. Thus, fluctuation of engine speed or the like can be preventedand satisfactory combustion characteristics of the engine can bemaintained.

Particularly when the reference position learning is performed with thetiming of completion of the reference position learning (time point t1of FIG. 15) intentionally shifted among the plurality of banks, it isadvantageous to have the valve timing setting matched among theplurality of banks at the time of timing control, through thedetermination as to whether the valve control may be started or not, asshown in FIG. 16.

When the reference position learning is executed every time the enginestarts, it is preferred to start reference position learning as soon aspossible after time point tb, when the engine speed is once settledafter the engine speed has increased by full combustion of the engineafter the start at time point ta, as shown in FIG. 17.

At the time of reference position learning, however, the intake valvephase is set uniformly without considering combustion characteristics ofthe engine. Therefore, when the reference position learning is executedduring an idle operation, combustion characteristics of the engine tendto deteriorate, possibly resulting in an engine stall in the worst case.

Further, generally, in the idle operation, idle speed control (ISC) isperformed for adjusting the amount of air introduced to the cylinder, inorder to maintain the engine speed at a prescribed idle speed. In theidle speed control, the basic amount of air, which is necessary tomaintain the target idle speed (in hot condition) in a stable state ofoperation with the engine being hot, is corrected (increased ordecreased) reflecting the difference in conditions between the stablestate of operation and the present state (such as engine temperature andoperation state of accessories). Typically, the amount of air to beintroduced under the idle speed control is adjusted by controlling openposition of throttle valve 1030.

In the present embodiment, when the reference position learning isexecuted during the idle operation, deterioration of combustioncharacteristics is prevented by the control structure in accordance withthe flowchart shown in FIG. 18.

Referring to FIG. 18, at step S200, ECU 4000 confirms whether thereference position learning is being executed or not, and at step S210,confirms whether the engine is in the idle operation or not.

When the reference position learning is executed during the idleoperation (YES at both steps S200 and S210), ECU 4000 increases, at stepS220, the amount of introduced air under idle speed control, so as toprevent an engine stall. Specifically, when the reference positionlearning is executed, the idle speed control is done with the amount ofintroduced air increased than when the reference position learning isnot executed, assuming that other conditions are the same.

As a result, deterioration of combustion characteristics leading to anengine stall can be prevented when the reference position learning isexecuted during an idle operation of the engine.

In the variable valve timing apparatus in accordance with the presentembodiment, the reference position may not be the most retarded angle,when a mechanism such as a lock pin is provided for mechanicallylimiting the change in the intake valve phase at the reference phase atthe time of reference position learning. It is possible, however, bysetting the reference phase in the reference position learning at thephase corresponding to the limit position of variable range of intakevalve phase (most retarded angle/most advanced angle) as in the presentembodiment, to execute the reference position learning without addingany special mechanism such as the lock pin.

In the embodiment described above, learning control portion 6100 of FIG.12 or steps S110 to S160 of FIG. 14 correspond to the “referenceposition learning means (step)” of the present invention, steps S160 andS170 of FIG. 16 correspond to the “learning completion confirming means(step)” of the present invention, and step S195 corresponds to the“control start limiting means (step)” of the present invention. Further,step S220 of FIG. 18 corresponds to the “increasing means (step)” of thepresent invention.

It should be understood that the embodiments disclosed herein areillustrative and non-restrictive in every respect. The scope of thepresent invention is defined by the terms of the claims, rather than thedescription above, and is intended to include any modifications withinthe scope and meaning equivalent to the terms of the claims.

1. A variable valve timing apparatus provided in an engine having aplurality of cylinder groups, comprising: a changing mechanism providedcorresponding to each of said plurality of cylinder groups, each forchanging a timing of opening/closing at least one of an intake valve andan exhaust valve in the corresponding cylinder group; reference positionlearning means provided corresponding to each said changing mechanism,for generating an operation command to an actuator of corresponding saidchanging mechanism so that said opening/closing timing is changed to aprescribed timing when reference position learning is instructed, andwhen said opening/closing timing reaches said prescribed timing, forlearning a reference of said opening/closing timing in response;learning completion confirming means for confirming whether saidreference position learning by said reference position learning meanshas been completed in every said changing mechanism or not; and controlstart limiting means for inhibiting, in each said changing mechanism,start of opening/closing timing control for changing saidopening/closing timing following a successively set target value, untilit is confirmed by said learning completion confirming means that saidreference position learning has been completed in every said changingmechanism.
 2. The variable valve timing apparatus according to claim 1,wherein each said changing mechanism is configured to change saidopening/closing timing by an amount of change in accordance with anoperation amount of said actuator, and further configured such that thechange in said opening/closing timing is mechanically limited at saidprescribed timing, at least during said reference position learning; andsaid reference position learning means generates an operation command ofsaid actuator in each said changing mechanism so that the timing atwhich the opening/closing timing reaches said prescribed timing differsfrom one said changing mechanism to another, when said referenceposition learning is instructed.
 3. The variable valve timing apparatusaccording to claim 1, further comprising increasing means forincreasing, when said reference position learning is executed while saidengine is in an idle operation, an amount of air introduced to saidengine to be larger than when said reference position learning is notexecuted under the same conditions.
 4. A variable valve timing apparatusprovided in an engine having a plurality of cylinder groups, comprisinga changing mechanism provided corresponding to each of said plurality ofcylinder groups, each for changing a timing of opening/closing at leastone of an intake valve and an exhaust valve in the correspondingcylinder group; and a control unit for controlling an operation of saidchanging mechanism; wherein said control unit is configured to executereference position learning independently in each said changingmechanism, to generate an operation command to an actuator ofcorresponding said changing mechanism so that said opening/closingtiming is changed to a prescribed timing when said reference positionlearning is instructed, and when said opening/closing timing reachessaid prescribed timing, to learn a reference of said opening/closingtiming in response, and said control unit is further configured toconfirm whether said reference position learning has been completed inevery said changing mechanism or not, and to inhibit, in each saidchanging mechanism, start of opening/closing timing control for changingsaid opening/closing timing following a successively set target value,until it is confirmed that said reference position learning has beencompleted in every said changing mechanism.
 5. The variable valve timingapparatus according to claim 4, wherein each said changing mechanism isconfigured to change said opening/closing timing by an amount of changein accordance with an operation amount of said actuator, and furtherconfigured such that the change in said opening/closing timing ismechanically limited at said prescribed timing, at least during saidreference position learning; and said control unit generates anoperation command of said actuator in each said changing mechanism sothat the timing at which the opening/closing timing reaches saidprescribed timing differs from one said changing mechanism to another,when said reference position learning is instructed.
 6. The variablevalve timing apparatus according to claim 4, wherein said control unitfurther generates a control command to increase, when said referenceposition learning is executed while said engine is in an idle operation,an amount of air introduced to said engine to be larger than when saidreference position learning is not executed under the same conditions.7. The variable valve timing apparatus according to claim 1, wherein ineach said changing mechanism, said prescribed timing is providedcorresponding to a mechanical limit of a variable range of saidopening/closing timing.
 8. The variable valve timing apparatus accordingto claim 1, wherein said actuator is implemented by an electric motor,and said changing mechanism is configured to change said opening/closingtiming by an amount of change corresponding to a difference in rotationspeed of said electric motor relative to the rotation speed of acamshaft driving the valve of which opening/closing timing is to bechanged.
 9. A method of controlling a variable valve timing apparatushaving, for each cylinder group, a changing mechanism for changing atiming of opening/closing at least one of an intake valve and an exhaustvalve provided in an engine, comprising: a reference position learningstep, executed for each said changing mechanism, of generating anoperation command to an actuator of corresponding said changingmechanism so that said opening/closing timing is changed to a prescribedtiming when reference position learning is instructed, and when saidopening/closing timing reaches said prescribed timing, and learning areference of said opening/closing timing in response; a learningcompletion confirming step of confirming whether said reference positionlearning has been completed at said reference position learning step, inevery said changing mechanism; and a control start limiting step ofinhibiting, in each said changing mechanism, start of opening/closingtiming control for changing said opening/closing timing following asuccessively set target value, until it is confirmed at said learningcompletion confirming step that said reference position learning hasbeen completed in every said changing mechanism.
 10. The method ofcontrolling a variable valve timing apparatus according to claim 9,wherein each said changing mechanism is configured to change saidopening/closing timing by an amount of change in accordance with anoperation amount of said actuator, and further configured such that thechange in said opening/closing timing is mechanically limited at saidprescribed timing, at least during said reference position learning; andat said reference position learning step, an operation command of saidactuator in each said changing mechanism is generated so that the timingat which the opening/closing timing reaches said prescribed timingdiffers from one said changing mechanism to another, when said referenceposition learning is instructed.
 11. The method of controlling avariable valve timing apparatus according to claim 9, further comprisingan increasing step of increasing, when said reference position learningis executed while said engine is in an idle operation, an amount of airintroduced to said engine to be larger than when said reference positionlearning is not executed under the same conditions.
 12. The method ofcontrolling a variable valve timing apparatus according to claim 9,wherein in each said changing mechanism, said prescribed timing isprovided corresponding to a mechanical limit of a variable range of saidopening/closing timing.
 13. The method of controlling a variable valvetiming apparatus according to claim 9, wherein said actuator isimplemented by an electric motor, and said changing mechanism isconfigured to change said opening/closing timing by an amount of changecorresponding to a difference in rotation speed of said electric motorrelative to the rotation speed of a camshaft driving the valve of whichopening/closing timing is to be changed.